1. Field of the Invention
This invention relates to antennas and, more particularly, to a practical implementation of a low-loss, high-efficiency, broadband antenna incorporating both electric and magnetic radiating components.
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
A wide operating frequency range is currently used for purposes of communications, especially ultra-wideband (UWB) communications and electromagnetic compatibility (EMC) testing. For example, many commercial and military-based communication devices operate within the 3 MHz to 30 MHz “high frequency” (HF) band, the 30 MHz to 300 MHz “very high frequency” (VHF) band, and in some cases, lower portions of the 300 MHz to 3 GHz “ultra high frequency” (UHF) band. Advantages of these relatively low frequency bands include improved diffraction around and penetration through obstacles, such as walls and foliage, and reduced path loss and attenuation in air, resulting in longer transmission lengths for a given power level. Due to the inverse relationship between size and operating frequency, antenna elements of relatively large size are often used for communicating in relatively low frequency bands, such as the HF and VHF bands. In many cases, however, it may be desirable for the antenna elements to be as small as possible for reasons of convenience, durability, space constraints and/or aesthetics.
Electrically-small antenna elements are utilized in many low frequency (e.g., mobile communications) and high frequency (e.g., EMC testing) applications. For example, an electrically-small antenna may be used in low frequency applications to accommodate space, durability or other concerns, or in high frequency applications to achieve a particular frequency level, which may be desired for EMC testing purposes. As used herein, the term “electrically-small” refers to an antenna or antennal element with relatively small geometrical dimensions when compared to the wavelengths of the electromagnetic fields they radiate. Quantitatively speaking, electrically-small antennas are generally defined as antennas which fit inside a sphere with a radius, a=λ/2π, where λ is the wavelength of the electromagnetic energy radiated from the antenna.
Unfortunately, electrically-small antennas tend to have rather large radiation quality factors (Q) meaning that they tend to store (on time average) much more energy than they radiate. This leads to input impedances that are predominantly reactive, which can make it difficult, if not impossible, to impedance match an electrically-small antenna to an input feed over a broad range of bandwidths. Furthermore, due to the large radiation quality factor, the presence of even small resistive losses may lead to very low radiation efficiencies in electrically-small antennas (e.g., around 1-50% efficiency).
According to known quantitative predictions, the minimum attainable radiation Q for any linearly-polarized, electrically-small antenna, which fits inside a spherical volume of radius, a, can be found by:
                    Q        =                              1            ka                    +                      1                                          k                3                            ⁢                              a                3                                                                        (                  EQ          .                                          ⁢          1                )            where k=1/λ, the wave number associated with the electromagnetic radiation. Thus, the radiation Q of an electrically-small antenna may be roughly proportional to the inverse of its electrical volume (a), or inversely proportional to the antenna bandwidth. In order to achieve relatively broad bandwidth and high efficiency with a single-element, electrically-small antenna of a given size, it may be desirable to utilize as much of the volume (that the antenna occupies) as possible. This may be achieved, in some cases, by increasing the size of the antenna elements, while retaining an electrically-small status.
In order to achieve the fundamental limit on radiation Q, as set forth in EQ. 1, an antenna would have to excite only the Transverse Magnetic (TM01) or Transverse Electric (TE01) mode outside of the enclosing spherical surface, and store no electric or magnetic energy inside the spherical surface. So while, a short linear (electric) dipole excites the TM01 mode outside of the sphere, it does not satisfy the criterion of storing no energy within the sphere, and thus, exhibits a higher radiation Q (and narrower bandwidth) than that predicted by EQ. 1.
In general, all antennas that radiate dipolar fields, such as electric and magnetic dipoles, are limited by the constraint given in EQ. 1. Though some broadband dipole designs have been successfully implemented and approach the limit given in EQ. 1, it is currently impossible to construct a linearly-polarized, omnidirectional antenna that exhibits a radiation Q less than that predicted by EQ. 1. However, while EQ. 1 represents the fundamental limit on the radiation Q of a linearly-polarized, omnidirectional antenna, it is not the global lower limit on radiation Q. Instead, a compound antenna which radiates substantially equal power into the TM01 and TE01 modes can (in principle) achieve a radiation Q of approximately:
                    Q        =                              1            2                    ⁡                      [                                          2                ka                            +                              1                                                      k                    3                                    ⁢                                      a                    3                                                                        ]                                              (                  EQ          .                                          ⁢          2                )            or roughly half that of an isolated electric or magnetic dipole, which radiates the TM01 or TE01 mode, alone. In other words, the impedance bandwidth of a compound antenna can be nearly double that of an isolated electric or magnetic dipole.
Ideal compound antennas having a pair of infinitesimally-small electric and magnetic dipoles, which are co-located and oriented to provide orthogonal dipole moments, have been theoretically and numerically examined and found to provide useful features. Such antennas are often referred to as “P×M antennas,” due to their orthogonal combination of electric (p) and magnetic (m) dipole vectors. Desirable characteristics of P×M antennas may include, but are not limited to, a useful radiation pattern (e.g., a low-gain, unidirectional radiation pattern) and a relatively broad impedance bandwidth for a given electrical size. As noted above, the radiation Q of an electrically-small P×M antenna is approximately half that of an isolated electric or magnetic dipole. Though the reduced Q should improve broadband impedance matching (at least in principle), practical implementations of P×M antennas have been problematic and have not been thoroughly investigated.